Abstract
Publisher Summary Viviparity is defined as the retention of an embryo in utero for the duration of embryonic development, resulting in the live birth of a fully developed neonate. Among vertebrates, squamate reptiles exhibit a significant number of transitions from oviparity to viviparity. The evolution of viviparity has been of considerable interest for over 100 years because of the high frequency of occurrence coupled with a complete absence of viviparity in other reptiles. In order for viviparity to fully evolve, viviparous species must overcome three major morphological and/or physiological modifications. First, egg retention time must be increased so that the embryo completes development in utero. Second, eggshell thickness must be decreased to reduce the physical barrier between embryonic and maternal environments. Third, a placenta must develop to facilitate maternal and embryonic exchange. This chapter provides a brief overview of some of the work that has shaped the understanding of these modifications associated with the evolution of viviparity. Further, a new hypothesis for the evolution of endocrine extraembryonic membranes is presented, including the placenta, of amniotes.
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